Abstract Hydrofluoric acid offers a fast etching rate for removing 200 μm-thick alpha-cases from the surfaces of cast Ti-6Al-4V. The acid picks up hydrogen gas into substrate of the cast, which can be limited by introducing an oxidizing acid. This study proposes the optimal ratios of hydrofluoric/nitric acid as the pickling solutions. The cast Ti-6Al-4V in these solutions is monitored with chemical etching and electrochemical polarization. The mechanisms of etching procedures are initiated by generating oxygen vacancies that lead to several products from a series of quasi-chemical steps. The final product of the reactions may deduce as the form of H 2TiF 6 that is peeled out by the evolution of produced gases. Polarized curves are performed to illustrate passive behaviors of the alpha-case removed Ti-6Al-4V. The amounts of charge transference increase with increasing hydrofluoric acid concentrations. The newly formed passive film was estimated to be 20 nm in thickness with non-stoichiometric arrangements in the 4% HF electrolyte.